The present invention relates to a laser source.
It is known that a laser source comprises, in general, at least:
an active element comprising at least one doped rod, in which a stimulated amplification (laser) phenomenon takes place;
a pumping system generating at least one pump beam, which is emitted into said active element so as to provide the energy needed for said laser amplification; and
an optical cavity which gives the laser beam obtained by this laser application its directivity and geometrical characteristics.
To obtain satisfactory pumping, it is known that it is necessary for the latter to have a high yield and to be highly uniform (over the doped rod of the active element), particularly in the case of a three-level laser.
With regard to the arrangement of the pumping system, a first type of pumping is known, called transverse pumping, which consists in placing said pumping system, generally a laser diode, orthogonally to the rod to be pumped. In this case, as long as the rod is not too long, it is possible to obtain good uniformity along the longitudinal axis of the latter. However, the yield is often low. Furthermore, when the rod is longer than the pumping system, it is necessary to provide an optical system to make the pumping uniform. Such an optical system is generally very bulky. Despite everything, the yield also remains low.
To increase the yield, it is possible to use a second type of pumping, called longitudinal pumping, which consists in placing the pumping system along the axis of the laser rod. In this case, although the yield may be very high, the uniformity is reduced since it is difficult to pump that end of the rod which is remote from the pumping system.
Furthermore, to make the pattern of pump energy symmetrical, it is general practice to inject the pump beams emitted by laser diodes into an optical fiber. At the fiber exit, the near-field and far-field profiles are circular, this being favorable to pumping. However, fiber-coupled laser diodes of this type are very expensive. In addition, the energy deposition obtained decreases strongly between the entry face, where the pump beams enter the rod, and the exit face. This reduces the yield.
The present invention relates to a laser source which makes it possible to remedy these drawbacks, that is to say a laser source whose pumping has both a high yield and high uniformity.
Document EP-0 404 635 discloses an illumination structure for a rod laser, having delocalized optical sources, enabling efficient removal of the thermal energy generated by the optical pumping sources, while still obtaining satisfactory uniformity of pumping. To do this, the optical sources are placed on the same delocalized support, which is provided with a heat sink, and the illumination structure includes an optical transfer system for transferring the pump beam to the rod. This optical transfer system consists of reflecting means which send the pump beam toward the rod.
However, this known illumination structure is relatively bulky and the yield is not optimal.
It is an object of the present invention to remedy these drawbacks. It relates to a compact laser source, the pumping of which is, in particular, highly uniform and has a high yield.
For this purpose, according to the invention, said laser source of the type comprising at least:
a pumping system (2) for generating at least one pump beam; and
an active element which comprises an elongate rod provided with a doped matrix capable of absorbing pump rays of said pump beam in order to amplify the laser radiation and at least one optical block placed on one side of said rod in order to guide the pump rays toward said rod, is noteworthy in that said pumping system is formed so as to generate a plurality of pump rays which are mutually parallel and distributed transversely to said active element, facing at least one entry face of said optical block, in that said entry face of the optical block is, at least partly, inclined with respect to the longitudinal axis of said rod and is, at least partly, non-orthogonal to the pump rays generated by the pumping system in order to deflect, by refraction, said pump rays so as to transmit them toward said rod and in that the inclination of said entry face is such that it deflects the pump rays in such a way that these rays reach said rod so that they are distributed over its entire length.
Thus, by virtue of the invention, the pump rays are uniformly distributed over the entire length of the rod in such a way that the pumping is particularly uniform.
In addition, the deflection of the pump rays is achieved by simple refraction. Consequently, it is unnecessary, in order to achieve this deflection, to provide specific means, such as reflection means for example, which are sometimes bulky and which require additional, often expensive and time-consuming treatments.
It should also be noted that, according to the invention, the entry of the pump rays into the optical block is different from the usual practice, whereby the pump rays are emitted orthogonally to the entry face of the optical block.
Preferably, said active element comprises two optical blocks which are placed on each side of said rod and are each provided with an inclined entry face in such a way that the pumping of the rod is carried out on both sides. In addition, this pumping is identical on both sides if the active element and the pumping system are symmetrical with respect to the longitudinal axis of said rod.
Thus, by virtue of these additional features, the pumping is uniform not only over the entire length of the rod, but also (everywhere) throughout its thickness.
It is known in fact that the pump energy is in general highly absorbed over a short distance of penetration into the rod so that that part of the rod lying on the opposite side from the region of penetration of the pump beam into said rod is usually weakly pumped. By virtue of the aforementioned pumping along both sides, this drawback is remedied and the pumping is made uniform over the entire rod (even if the center of the rod is, nevertheless, slightly less pumped than its edges).
Furthermore, by virtue of the invention, the pump rays necessarily pass through the rod, thereby making it possible to obtain a high yield.
Advantageously:
the pumping system comprises at least one linear array of laser diodes; and
said pump rays are generated parallel to the longitudinal axis of said rod by said pump system.
In addition, advantageously, the inclination of each entry face depends on the length L of the rod and on the refractive index n of the medium forming the optical block with respect to the medium in which the pump rays are generated. More precisely, in an advantageous manner, the angle of inclination (xcex8) between the inclined entry face and the longitudinal axis of the rod approximately satisfies the following equation:
cosxcex8=n.cos(xcex8+arctan(H/L))
in which:
cosxcex8 represent the cosine of xcex8;
arctan(H/L) represents the inverse tangent of H/L; and
H is the width of the optical block orthogonal to the longitudinal axis of said rod at the entry face.
Moreover, in one particular embodiment, each optical block has, on the opposite side from its entry face, a second inclined face and the inclinations of said second faces are such that they cause the pump rays to return (toward the entry faced), generating at least one additional pass through said rod. Thus, the number of passes through the rod is increased for the same pump ray, thereby increasing the efficiency of the pumping.
Furthermore, in an advantageous manner, the rod has a rectangular cross section, the two adjacent sides of which define the width and the height of the rod respectively, said height corresponds to the height of the active element, the width of the active element comprises said width of said rod and the widths of the optical blocks and the height of the active element is less than twice the width of the rod. By virtue of this reduced height of the active element (between an upper face and a lower face) compared with its width, it is possible to guide the pump rays, making them be reflected off said upper and lower faces. This makes it possible to define a short guide path and thus rapid guiding.
Moreover, with regard to the refractive index, the index of each optical block is advantageously less than the index of the rod. Thus, the pump rays are prevented from being reflected by the rod, instead of passing through it.